Solid state radiation imager with high integrity barrier layer and method of fabricating

ABSTRACT

A radiation imager includes a photosensor barrier layer disposed between an amorphous silicon photosensor array and the scintillator. The barrier layer includes two strata, the first stratum being silicon oxide disposed over the upper conductive layer of the photosensor array and the second stratum is silicon nitride that is disposed over the first stratum. The photosensor barrier layer has a shape that substantially conforms to the the shape of the underlying upper conductive layer and has a maximum thickness of about 3 microns. The silicon oxide and silicon nitride are deposited in a vapor deposition process at less than about 250° C. using tetraethoxysilane (TEOS) as the silicon source gas.

RELATED APPLICATIONS

This application is a continuation-in-part of the application of R. F.Kwasnick and J. D. Kingsley entitled "Photosensitive Element with TwoLayer Passivation Coating", Ser. No. 07/891,117 (RD-21,721), filed 1Jun. 1992 now U.S. Pat. No. 5,233,181, which is assigned to the assigneeof the present invention and incorporated herein by reference.

BACKGROUND OF THE INVENTION

Solid state imagers typically include a photosensor array coupled to ascintillating medium. Radiation absorbed in the scintillator generatesoptical photons which in turn pass into a photosensor, such as aphotodiode, in which the optical photon is absorbed and an electricalsignal corresponding to the incident optical photon flux is generated.Substantially hydrogenated amorphous silicon (a-Si) is commonly used inthe fabrication of photosensors due to the advantageous photoelectriccharacteristics of a-Si and the relative ease of fabricating suchdevices. In particular, photosensitive elements, such as photodiodes,can be formed in connection with necessary control or switchingelements, such as thin film transistors (TFT's), in a relatively largearray. In such arrays both the TFT's and the photodiodes typicallycomprise a-Si.

The performance of amorphous silicon based imagers can be degraded by anumber of factors, including, for example, exposure to moisture (whichcan cause the leakage of a-Si photodiodes to increase irreversibly),exposure to materials, such as solvents used in the fabrication process,that degrade the electrical characteristics of the a-Si photodiode ormay damage the polymeric dielectric materials, or by exposure totemperatures higher than those of the a-Si deposition processes (e.g.,greater than about 250° C.). It is thus beneficial to use fabricationprocesses that do not present adverse environmental conditions and toprovide a protective boundary for a-Si components to minimizedegradation both in fabrication and during operation of the imager. Aprotective boundary disposed between the a-Si components of thephotosensor array and the scintillator in an imager desirablyprovides 1) protection of the photosensor array from contamination bythe scintillating medium (and vice versa);2) a good surface to which thescintiilator material can adhere;3) good optical coupling between thescintillator and the underlying pixels in the imager array (that is, ahigh degree of transmission of the optical photons with a minimum ofscattering of the photons);and,4) a good environmental barrier toprotect the photosensor array, especially from moisture.

In U.S. Pat. No. 4,906,850 to Beerlage, a thick layer of dielectric,such as silicon nitride or silicon oxide, is deposited over thephotosensor array such that it can be patterned to form a number ofislands on which a scintillator can be vapor deposited so that it growsup from the upper surfaces of these islands, In the Beerlage device, thegrooves cut into the silicon oxide or silicon nitride layer to form theislands are about 10-20 μm wide and 10-20 μm deep, which necessarilyimplies that the overall protective layer is quite thick (i.e., greaterthan at least 10 μm). Similarly, the growing scintillator columns fromsuch a large area (i.e., the relatively large top surface of the islandsformed in the silicon oxide or silicon nitride) necessitates the use ofa high-temperature scintillator deposition process (e.g., about 250° C.)that exposes the a-Si array components and the dielectric material tothe same high temperatures. Thick silicon oxide or silicon nitridelayers present other problems in imager arrays. For example, such thicklayers are prone to crack (both vertically and horizontally), causingpoor optical transmission characteristics, such as scattering of lightcaused by crack channels, and the thick layers are prone to delaminate,causing structural degradation of the imager.

It is thus an object of this invention to provide an imager array inwhich the a-Si components are protected from environmental elements,such as moisture.

It is a further object of this invention to provide an imager arrayhaving a high degree of protection from environmental elements and thatis mechanically robust, providing good adhesion between the imager arrayand the scintillator.

It is a still further object of this invention to provide a method offabricating an imager array to provide a protective layer and thatminimizes exposure to the array to high temperatures in the fabricationprocess.

SUMMARY OF THE INVENTION

In accordance with this invention a solid state imager comprises aphotosensor array, a scintillator optically coupled to the photosensorarray, and a photosensor array barrier layer disposed therebetween. Thephotosensor barrier layer is disposed over the upper conductive layer ofthe photosensor array and is relatively thin, having a maximum thicknessof less than about 3 μm or less, and a typical thickness of abut 1 μm.The shape of the array barrier layer conforms to the contour of theupper conductive layer and comprises silicon nitride. The array barrierlayer typically comprises first and second strata (or layers), with thefirst stratum being disposed over the upper conductive layer of thephotosensor array and comprising silicon oxide having a thickness in therange of between about 0.5 μm and 1.5 μm and the second stratum beingdisposed over the first stratum and having a thickness of between about0.05 μm and 0.15 μm. In one embodiment of the present invention apassivation layer is disposed between the a-Si photosensor body and theupper conductive layer, the passivation layer comprising an inorganicmoisture barrier layer and an organic dielectric layer.

A method of fabricating an imager array in accordance with thisinvention includes the steps of forming a photosensor array having anupper conductive layer and depositing a barrier layer over thephotosensor array such that the barrier layer substantially conforms tothe shape of the upper conductive layer and has a maximum thickness ofabout 3 μm. The step of forming the barrier layer further includes thestep of depositing a first stratum of silicon oxide and a second stratumof silicon nitride thereover, the silicon oxide being deposited at atemperature less than about 250° C. The step of depositing the siliconoxide further includes the step of applying the silicon oxide usingtetraethoxysilane (TEOS) as the silicon source gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description in conjunction with the accompanying drawingsin which like characters represent like parts throughout the drawings,and in which the sole FIGURE is a cross-sectional view of an imagerarray formed in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A radiation imager 100 as illustrated in the FIGURE comprises asubstrate 110 on which a photosensor array 120 is disposed, ascintillator 180 that is optically coupled to photosensor array 120,and, in accordance with this invention, a photosensor array barrierlayer 150 disposed between photosensor array 120 and scintillator 180.

Photosensor array 120 comprises a plurality of photosensitive elementsthat are typically arranged in rows and columns on substrate 110.Alternatively, photosensor array 120 may be disposed over a dielectriclayer (not shown) that covers substrate 110, as might be the case ifother components are disposed on substrate 110 below the photosensorarray. As used herein, "lower" or "bottom" are used interchangeably torefer to a relative position of a portion of imager array 100 closer inproximity to substrate 110, and "upper" or "top" are usedinterchangeably to refer to a relative position of a portion of imagerarray closer farther from substrate 110; no operational or functionallimitation on the orientation of the imager array is implied. Eachphotosensitive element typically comprises a photodiode 130 comprising aphotosensor island 132 disposed over a bottom contact pad 134 and withan upper conductive layer 136 disposed over the assembly. Upperconductive layer comprises a substantially transparent conductivematerial such as indium tin oxide or the like.

Photosensor island 132 comprises light absorptive semiconductivematerial such as substantially intrinsic hydrogenated amorphous silicon(a-Si), and may comprise bands (not shown) of silicon doped to show aselected conductivity (i.e., n-type or p-type) to provide the desiredrespective electrical contact to bottom contact pad 134 and upperconductive layer 136. Photosensor island is disposed between upperconductive layer 136 and bottom contact pad 134 such that a selectedbias voltage is applied across the photosensor body. Consequently,charge generated by the absorption of optical photons in thesemiconductive material is collected at a selected electrode that isperiodically "read" or measured, at which time the bias voltage acrossthe photodiode is reset to its selected value.

Each photodiode 130 is coupled to circuitry to allow charge collected onthe photodiode to be read. Typically each photodiode is selectivelycoupled to a data line 125 by a respective thin film transistor (notshown); a dielectric layer 127 typically overlies the plurality of TFT'sand data lines and provide electrical insulation between thesecomponents and overlying components, such as photodiode 130.

Photosensor array 120 advantageously comprises a passivation layer 140which is disposed under upper conductive layer 136 except at pointswhere upper conductive layer 136 is disposed in electrical contact withthe upper surface of photosensor island 132. Passivation layer 140comprises an inorganic moisture barrier layer 142 and an organicdielectric layer 144. Inorganic layer 142 typically comprises siliconnitride and is relatively thin, between about 0.01 micron and 0.5micron; organic dielectric layer 144 typically comprises a polyimide,preferably a preimidized polyimide such as the Ciba-Geigy® 200 series,and has a thickness between about 0.5 micron and 2.5 micron. Detailspertaining to passivation layer appear in the U.S. Pat. No. 5,233,181,incorporated herein by reference.

In accordance with this invention, photosensor barrier layer 150 isconformably disposed over upper conductive layer 136 and has a maximumthickness of about 3 microns or less. As used herein, "conformablydisposed," "conformal," or the like refers to photosensor barrier layer150 having a shape that is substantially similar to the contour of upperconductive layer 136, such that passivation layer 150 is disposed onupper conductor 136 and has a substantially uniform thickness along itslength. Scintillator 180 is disposed over barrier layer 150 and isoptically coupled to photosensor array 120 through the barrier layer.Barder layer 150 is adapted to provide good optical coupling of photonsbetween scintillator 180 and photosensor array 120 (that is, opticalcoupling with a high degree of transmission and with little or noscattering of the optical photons such as is provided by transparentdielectric materials without cracks). Barrier layer 150 is furtheradapted such that the scintillator material adheres well to the uppersurface of barrier layer so that there is little or no mechanicaldeterioration of the bond between scintillator 180 and barrier layer 150under normal operating conditions for the imager; e.g., the structureshould pass the standard tape pull test (wherein the adhesive tape isapplied to the structure and pulled off, as is known in the art) withoutdelamination. Further, barrier layer 150 is relatively thin such that innormal operating environments in which imagers are used the barrierlayer remains substantially crack free and does not delaminate.Commonly, barrier layer 150 has a thickness of about 1 μm.

Photosensor array barrier layer 150 typically comprises a first stratum(or layer) disposed adjacent to upper conductive layer 136 and a secondstratum 156 disposed on first stratum 152, with scintillator 180 beingdisposed on second stratum 156. First stratum 152 comprises siliconoxide that has a typical thickness in a range between about 0.5 micronsand 1.5 microns, and commonly has a thickness of about 0.7 microns. Thesilicon oxide comprising first stratum 152 is typically deposited in aplasma enhanced chemical vapor deposition (PECVD) process usingtetraethoxysilane (TEOS) as the silicon source gas at a temperature ofless than about 250° C., providing a silicon oxide having a compositionof SiO_(x), wherein 1.5≦x≦2, and hydrogen content is less than about 5%of the atoms. Silicon oxide deposited in this process conforms well tothe underlying layer (i.e., step coverage is good over the contours ofupper conductive layer 136) even while maintaining a depositiontemperature of about 210° C. Additionally, the silicon oxide provides arobust moisture barrier and is resistant to solvents, such as gammabutyrolactone, which may be present in the array from the deposition ofpolyimide.

Second stratum 156 comprises silicon nitride having a typical thicknessin a range between about 0.05 μm and 0.15 μm, and commonly has athickness of about 0.1 μm. The silicon nitride comprising second stratumis deposited in a PECVD process at a temperature of about 210° C., andtypically at a reduced frequency of about 50 KHz (as opposed the morecommonly used PECVD frequency of 13.56 MHz). This process providessilicon nitride having a composition of SiN_(y), wherein 1.0≦y≦1.33, andhydrogen content is less than about 10% of the atoms. The siliconnitride of second stratum 156 additionally provides a surface adapted toprovide good adhesion to the scintillating medium deposited thereover.

In an alternative embodiment of this invention (not shown), photosensorbarrier layer comprises only one stratum, which comprises siliconnitride. Such a layer typically has a thickness of between about 0.2 μmand 1 μm; except as noted herein, the device of the alternativeembodiment is otherwise the same as that described elsewhere in thespecification with respect to the two strata photosensor barrier layerdevice.

In the two strata barrier layer, typically the optical index ofrefraction of the silicon oxide first stratum is in the range between1.4 and 1.5, and the optical index of refraction of the silicon nitrideis in the range between about 1.8 and 2.3, and typically is about 2.0.Optical coupling between scintillator 180 and photosensor array 120through two-strata barrier layer 150 has been measured to be comparableto the optical coupling achieved with a single layer of silicon nitride.

Scintillator 180 comprises a scintillating medium adapted to emitoptical photons in response to the absorption of incident radiation ofthe type to be imaged; for example, for x-ray imagers, cesium iodidedoped with thallium (CsI:TI) is commonly used. The CsI:TI isadvantageously deposited on photosensor barrier layer 150 by evaporationin the form of needles 185; each needle is typically several microns indiameter and several hundred microns long. This aspect ratio in whichthe needle has a relatively long longitudinal axis compared to arelative short diameter provides a scintillator in which most opticalphotons emerge from the bottom of the scintillator (towards thephotosensor array) such that they are efficiently optically coupled intothe photosensor array; that is the majority of photons strike thephotosensor barrier layer at sufficiently large angles of incidence sothat substantially all photons pass into photosensor array 120 withoutbeing reflected at any of the following interfaces: between thescintillator material and the silicon nitride in second stratum 156;between the silicon nitride and the silicon oxide in first stratum 152;and between the silicon oxide and the upper conductive layer 136.

In accordance with this invention, in the fabrication process thesilicon oxide and silicon nitride layers of photosensor barrier layer150 may be deposited during the same vacuum pumpdown. It is furtherdesirable to deposit the silicon oxide for first stratum 152 and thesilicon nitride for second stratum 156 through a shadow mask toselectively deposit it only over the desired portion of upper conductivelayer 136 so that the silicon oxide and silicon nitride are notdeposited on areas, such as contact fingers, from which it must latermay be removed in an etching process that may also result in theundesired removal of other layers of silicon oxide or silicon nitrideforming portions of other components in imager array 100.

The process of forming a radiation imager in accordance with thisinvention includes the steps of forming a photosensor array on asubstrate, the photosensor array having an upper conductive layerdeposited thereover; depositing a photosensor barrier over the upperconductive layer such that, as noted above, the barrier layer conformsto the upper conductive layer and has a maximum thickness of about 3 μmor less. The barrier layer is formed from a first stratum comprisingsilicon oxide deposited at a temperature less than about 250° C.(typically about 210° C.) and using TEOS as the silicon source gas. Thesecond stratum of silicon nitride is deposited over the first stratum ina PECVD process using a gas mixture of silane, ammonia, and nitrogen.The silicon oxide and silicon nitride are advantageously depositedduring a single vacuum pumpdown of the array being fabricated, and TEOSis advantageously used as the silicon source gas for the silicon oxide.The step of forming a photosensor array advantageously comprises thesteps of forming a plurality of amorphous silicon photosensitive bodies;forming a passivation layer over the plurality of photosensitive bodies,the passivation layer comprising an inorganic moisture barrier layer andan organic dielectric layer; and depositing the upper conductive layerover the passivation layer and the photosensitive bodies.

A radiation imager formed in accordance with this invention exhibits arobust structure that is resistant to mechanical degradation and that isresistant to attack by moisture. For example, a comparison of theincrease in number of defective pixels in imager arrays operated in ahigh humidity environment (about 60-80% relative humidity (RH)) over aperiod up to 100 hours indicates that a photosensor array having abarrier layer fabricated in accordance with this invention (but with noscintillator disposed thereover) experienced much less degradation inthis inhospitable operating environment than imagers without a barrierlayer. Degradation was characterized by the number of new defectivepixels that appeared in the array after exposure to the testenvironment, a defective pixel being one in which the leakage exceeded 1pA with about 6 volts reverse bias across the photodiode.

The table below presents representative data from tests run with imagerarrays having varying diode passivation and barrier layer schemeswithout scintillator coatings. Each array comprises 40,000 pixels (eachhaving an area of about 4×10⁻⁴ cm²) that were scanned with thephotodiodes under reverse bias of about 8 volts to simulate operationalconditions in high humidity (about 60-80% RH) at an ambient temperatureof about 70° F.

Data for arrays having no photodiode passivation other than polyimide(PI) (designated "A-₋₋ "); having silicon nitride under the polyimide(designated "B-₋₋ "); having a two-strata barrier layer (designated"C-₋₋ "); and a single stratum barrier layer (designated "D-₋₋ ") arepresented:

    ______________________________________                                                                             Number of                                      Diode                  Scanning                                                                              Additional                                     Passivation:           Time at 60-                                                                           Defective                                Imager                                                                              SiNy under PI                                                                             Barrier Layer                                                                            80% RH  Pixels                                   ______________________________________                                        A-1   None        None       38 hrs. 280                                      A-2   None        None       32 hrs. 5021                                     A-3   None        None       31 hrs. 856                                      B-1   1000 Å  None       38 hrs. 54                                       B-2   1000 Å  None       41 hrs. 70                                       C-1   1000 Å  1350 Å SiN.sub.y /                                                                   46 hrs.  1                                                         7500 Å SiO.sub.x                                        C-2   1000 Å  1350 Å SiN.sub.y /                                                                   74 hrs.  8*                                                        7500 Å SiO.sub.x                                        C-3   1000 Å  1350 Å SiN.sub.y /                                                                   79 hrs.  15**                                                      7500 Å SiO.sub.x                                        D-1   1000 Å  1350 Å SiN.sub.y                                                                     39 hrs. 58                                       D-2   1000 Å  1350 Å SiN.sub.y                                                                     48 hrs. 22                                       D-3   1000 Å  2750 Å SiN.sub.y                                                                     46 hrs.  8                                       D-4   1000 Å  4150 Å SiN.sub.y                                                                     45 hrs. 12                                       D-5   1000 Å  6950 Å SiN.sub.y                                                                     49 hrs.  5                                       D-6   1000 Å  6950 Å SiN.sub.y                                                                     48 hrs.  5                                       ______________________________________                                         Notes to the Table:                                                           *No data taken on lot C2 in the 40-50 hour time frame.                        **In lot C3, after 94 hours and drying to obviate effects of                  moisturemediated lineto-line leakage, 2 or less pixels were found to be       degraded by the exposure to the high humidity environment. The number of      defective pixels noted at 79 hours is therefor believed to result from        moisture mediated line to line leakage as opposed to moisture penetration     and degradation of the pixels.                                           

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

What is claimed is:
 1. A solid state imager comprising:a photosensorarray having an upper conductive layer disposed substantially thereover,said upper conductive layer having an undulating surface topographycorresponding to the contour of the underlying photosensor arraycomponents; a photosensor array barrier layer comprising a first stratumdisposed over said upper conductive layer, and a second stratum disposedover said first stratum, said first stratum comprising silicon oxide andsaid second stratum comprising silicon nitride, said barrier layerhaving a shape conforming to the underlying conductive layer and havinga maximum thickness of less than about 3 μm; and a scintillator disposedover said photosensor array barrier layer and optically coupled to saidphotosensor array through said barrier layer; said first stratumproviding a robust moisture barrier and said second stratum providing anadhesion bonding surface for said scintillator.
 2. The imager of claim 1wherein said photosensor array comprises amorphous silicon.
 3. Theimager of claim 1 wherein the thickness of said silicon oxide layer isin the range between about 0.5 μm and 1.5 μm.
 4. The imager of claim 1wherein the thickness of said silicon nitride layer is in the rangebetween about 0.05 μm and 0.15 μm.
 5. The imager of claim 1 wherein saidphotosensor array comprises a plurality of photosensor bodies and apassivation layer disposed between said upper conductive layer and eachrespective photosensor body, said passivation layer comprising aninorganic moisture barrier layer and an organic dielectric layer.
 6. Theimager of claim 5 wherein said inorganic moisture barrier layercomprises silicon nitride and said organic dielectric layer comprisespolyimide.
 7. The imager of claim 1 wherein the thickness of saidbarrier layer is not greater than about 1 μm.
 8. The imager of claim 1wherein said upper conductive layer comprises indium tin oxide.
 9. Theimager of claim 1 wherein said silicon oxide in said first stratumcomprises SiO_(x), wherein 1.5≦x≦2.
 10. The imager of claim 1 whereinsaid silicon nitride in said second stratum comprises SiN_(y), wherein1≦y≦1.33.
 11. The imager of claim 1 wherein said silicon oxide in saidfirst stratum has an optical index of refraction between about 1.4 and1.5.
 12. The imager of claim 1 wherein said silicon nitride in saidsecond stratum has an optical index of refraction between about 1.8 and2.3.
 13. The imager of claim 1 wherein said scintillator comprisescesium iodide, said cesium iodide being disposed in a plurality ofneedle-shaped structures disposed over said upper conductive layer. 14.A radiation imager for detecting optical photons generated by absorptionof incident x-rays, comprising:a photosensor array having a plurality ofamorphous silicon photosensitive bodies disposed on a substrate and anupper conductive layer disposed thereover, said array further comprisinga passivation layer disposed between said upper conductive layer andsaid plurality of photosensitive bodies, said passivation layercomprising an inorganic moisture barrier layer and an organic dielectriclayer; a needle-structure scintillator optically coupled to saidphotosensor array such that optical photons generated in saidscintillator pass into said photosensor array; and a photosensor arraybarrier layer conformably disposed between said photosensor array andsaid scintillator, said array barrier layer having a maximum thicknessless than about 3 μm and being adapted to be substantially impervious tomoisture and optically non-diffusive; said array barrier layercomprising a first and a second stratum, said first stratum of saidarray barrier layer being disposed adjacent to said upper conductivelayer and said second stratum is disposed thereover, said first stratumcomprising silicon oxide and said second stratum comprising siliconnitride.
 15. The radiation imager of claim 14 wherein said first stratumhas a thickness in the range between about 0.5 μm and 1.5 μm.
 16. Theradiation imager of claim 15 wherein said second stratum has a thicknessin the range between about 0.05 μm and 0.15 μm.
 17. The radiation imagerof claim 16 wherein said upper conductive layer comprises indium tinoxide.